1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a method and apparatus for performing a handover of a user equipment in connected mode during discontinuous reception operation in a mobile communication system.
2. Description of the Related Art
The UMTS (Universal Mobile Telecommunication Service) system is a 3rd generation asynchronous mobile communication system that is based on European Mobile Communication Systems, i.e., GSM (Global System for Mobile Communications) and GPRS (General Packet Radio Services), and employs a WCDMA (Wideband Code Division Multiple Access) scheme. The 3GPP (3rd Generation Partnership Project) responsible for UMTS standardization is currently discussing LTE (Long Term Evolution) as a next generation UMTS system.
In the conventional 3rd generation mobile communication system, that is, the UMTS system, one way to minimize the power consumption of a user equipment (UE) in an idle mode is discontinuous reception (DRX) operation in which the UE discontinuously receive a paging message. The idle mode UE turns on its transceiver, wakes up from a sleep state, and monitors a paging channel at a predetermined point in time. If a paging message specific to the UE is received over the paging channel, the UE transitions to a connected mode. In contrasts, if a UE-specific paging message is not received over the paging channel, the UE turns off its transceiver, and maintains the sleep state until it wakes up next time.
In this way, the conventional 3rd generation mobile communication system applies the DRX operation of the UE only to the paging message. The reason for this is that voice communication is the first consideration in the conventional mobile communication system, and thus the period of time during which the UE stays in the connected mode is relatively short.
However, a new evolved mobile communication system, such as LTE, is expected to provide a UE with a voice packet service as well as a voice communication service. Since such a UE that is provided with the voice packet service may stay in a connected mode for a relatively long period of time, the problem of minimizing the power consumption of the UE in the long connected mode needs to be fundamentally solved.
In the evolved mobile communication system, a network may appropriately set the DRX operation for the connected mode UE by considering the characteristics of a service that is being provided.
However, when the DRX operation is set for the connected UE in this way, the UE's handover may deteriorate due to the DRX operation. In other words, the UE in the sleep state cannot receive a handover command from a base station during the DRX operation, resulting in the UE to finally fail in a handover.
Therefore, there is a need for a scheme to provide timely handover, which can maximize the system performance of a connected mode UE while minimizing the power consumption of the UE during DRX operation in the evolved mobile communication system.